Biodiesel is a non-toxic, carbon neutral renewable fuel which is generally produced by transesterification of oil feedstock with short chain alcohols and exhibits similar physical and chemical properties to conventional diesel fuel. Biodiesel may curtail the harmful emissions such as NOx, SOx, CO, CO2, unburnt hydrocarbon and particulates.
The conventional production of biodiesel through transesterification involves the breaking down of triglyceride molecule with alcohols to yield biodiesel and glycerol with presence of catalyst. Conventional production of biodiesel often involves a homogeneous strong acid (e.g. HCl or H2SO4) or strong base (e.g. KOH or NaOH).
The production cost for the transesterification of refined plant oil is 1.5 times higher than that of the conventional diesel fuel. It is advantageous that the development of biodiesel production from waste oil or low grade feedstock through transesterification directly would reduce the production cost. The waste oil and low grade feedstock often contain high degree of free fatty acids (FFAs) and water content which hinder the transesterification process. The high water content in low grade feedstock may result in hydrolysis of triglyceride to produce FFAs while the FFAs would subsequently react with the base catalyst to yield soaps that complicate the separation of glycerol from the catalytic system and thus, it suppresses the transesterification reaction.
To overcome this issue, two-step biodiesel production is applied to the low grade feedstock or waste oil. The first step of the two-step operation involves the pre-treatment step of FFAs removal by an acidic catalyst (e.g. HCl or H2SO4) followed by a second step of alkaline (e.g. KOH or NaOH) catalyzed transesterification. These homogeneous strong acid and base are highly corrosive and require a large amount of fresh water for biodiesel purification to generate an enormous amount of waste water and thus, the production cost would be increased. Although this two-step operation may utilize the low grade feedstock and waste oil, the process requires multi-steps washing and leads to loss of catalyst. The development of a new class of heterogeneous catalyst system that are active in one-step simultaneous esterification and transesterification is beneficial to increase the flexibility for various low grade feedstock selected in biodiesel production and reduce production cost.